A dimension-hopping D&D game, possibly in which all the characters are servitors and other spirits of the dead (giving them the neat advantage of being Outsiders and therefore unkillable under normal circumstances). Characters may be working their way up the Angelic hierarchy, trying to contact the world of the living, or have their own strange missions.
A game where everyone plays wizard's familiars. This will probably require designing a rule system for non-anthropomorphic, magic-wielding animals.
A Tick-style super hero game--though I'd much rather play in one!
A modern dream-exploration game where the characters real lives and sleeping worlds were equally important